Transistors are common components of many integrated circuits used in electronic devices, such as computers, mobile phones, televisions, etc. Organic devices, such as organic thin film transistors (OTFTs) are becoming increasingly popular for use in future electronic devices due to lower production costs and due to other advantages. For instance, OTFTs can be manufactured using roll-to-roll processing, ink jet printing, spray-on techniques, and are able to be deposited on flexible substrates.
Certain organic devices, such as OTFTs, have at least three terminals including a source, a drain, and a gate. Application of a bias voltage to the gate controls the amount of charge carriers that flow between the source and the drain. When a sufficient gate bias voltage is applied, charge carriers accumulate such that a highly conductive channel is created between the source and drain. In certain organic devices, the highly conductive channel is formed in an organic semiconductor layer of the organic device. A voltage bias between the source and drain drives the current flowing in the conductive channel between the source and drain. The organic device typically includes a threshold voltage that constitutes the minimum gate voltage necessary to allow current to flow between the source and drain. If the voltage applied to the gate is less than the threshold voltage, only nominal current flows between the source and drain.
Organic devices, such as OTFTs, can suffer from device stability issues such that the devices lose reliability of performance over time. For instance, application of a gate voltage for extended periods of time can lead to reduced source drain current throughput. The deleterious bias stress effects from application of the gate voltage can happen within seconds and can cause continuous degradation for hours to even days.
FIG. 1 provides a graphical representation of the reduced performance of an exemplary OTFT device over time due to application of a gate voltage. In particular, FIG. 1 illustrates the steady state response of an exemplary OTFT device with a gate voltage Vg of about −40V and a source drain bias VSD of about −20V. As illustrated by curve 100, the magnitude of the source drain current ISD substantially decreases with the passage of time due to bias stress effects resulting from the application of the gate voltage. The threshold voltage of the OTFT can also shift overtime, leading to reduced performance and reliability of the device.
An organic device can recover its original performance characteristics by removing the gate bias voltage for a period of time. The recovery time for the organic device, however, can often impose undesirable operating limits on the organic device. For instance, the recovery time can take several minutes to several hours depending on the amount of time the gate bias voltage was applied to the organic device. Moreover, the need to periodically remove a gate bias voltage from the organic device for a sufficient period of time to allow the organic device to recover its original performance characteristics can pose a barrier to wide range application of the organic devices.
The photoresponse of organic devices, such as OTFTs is known. For instance, the effects on drain current and threshold voltage of an exemplary OTFT as a result of illuminating the OTFT with monochromatic light at varying wavelengths and intensities are set forth in Michael C. Hamilton and Jerry Kanicki, “Organic Polymer Thin-Film Transistor Photosensors,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 10, No. 4, July/August 2004. Such techniques, however, fail to address reduction of gate bias voltage effects on the drain current while the gate bias voltage is applied to an organic device over time.
Thus, a need exists for a solution to reduce deleterious effects of organic devices as a result of gate bias stress. A solution that can provide for predictable control of the drain current response such that the organic device can be placed into multiple modes of operation on command would be particularly useful.